In mixtures of gas, such as natural gas, gas accompanying petroleum, cracked gas, synthesis gas, coke-oven gas, coal-gasification and distillation gas, it is in many cases necessary and desirable, to remove therefrom acid gases, such as carbon dioxide and hydrogen sulphide, and to separate specific hydrocarbon fractions. The removal of the acid gases is necessary because, as a rule, they interfere with the further processing thereof for a wide range of industrial purposes. Carbon dioxide is at least a ballast substance which requires inefficient enlargement of the equipment and which must frequently be removed because of subsequent processing requirements. Hydrogen sulphide is a noxious substance which cannot simply be released into the atmosphere and which, if it is used for burning may lead to inadmissible concentrations of sulphur dioxide in the exhaust gases. The purpose of removing various hydrocarbon fractions, especially if natural gas is to be used for burning, is to recover the considerable concentration of higher hydrocarbons frequently present in such gases. They may be used for engine propellants or as the starting materials for chemical syntheses of other petro-chemical products.
German Pat. No. 971.786 discloses a variety of methods of conditioning mixtures of gas containing not only hydrocarbons, but also carbon dioxide and hydrogen sulphide. It is suggested that sulphur compounds, C.sub.3 -to C.sub.5 -hydrocarbons, carbon dioxide, water vapour and resin-formers can be scrubbed in one operation from natural gases with the aid of inorganic, neutral, polar solvents, at temperatures as low as -150.degree. C. It is furthermore proposed to scrub out polar contaminants, at about -40.degree. C., with methanol as a solvent and then to dissolve out the C.sub.3 - to C.sub.4 -hydrocarbons with a mixture of C.sub.5 -to C.sub.7 -hydrocarbons. It has been found, however, that the polar scrubbing agent also dissolved undesirably large amounts of C.sub.3 - and C.sub.4 -hydrocarbons and, as a remedy for this, water was added to the methanol.
If it is particularly important to wash out C.sub.3 or higher hydrocarbons, German Pat. No. 971.786 suggests a further modification of the method, whereby the hydrocarbons are first separated with the aid of a neutral unpolar solvent to which is added a small amount of a neutral polar solvent. After this, the polar contaminants are removed from the gas, especially hydrogen sulphide and carbon dioxide, in a subsequent scrubbing stage, at a low temperature, with a lower alcohol. Water may be added to the lower alcohol to reduce the solubility of methane and ethane in the solvent. Further, it is proposed, in German Pat. No. 971,786, to cool the gas containing the hydrocarbons until pentane and the higher hydrocarbons condense out and then to scrub the C.sub.3 - and C.sub.4 -hydrocarbons, together with the H.sub.2 S and CO.sub.2, out of the gas with a neutral polar solvent.
However, all of these known methods have the disadvantage that the polar compounds, carbon dioxide and hydrogen sulphide, are always removed exclusively by scrubbing operations with considerable quantities of solvent, in part even together with considerable amounts of hydrocarbons. This is extremely disadvantageous, particularly in view of the thermal loading and the cost of regenerating the solvent.
It is, therefore, an object of the present invention to provide a method and an apparatus for conditioning or treating mixtures of gas containing hydrocarbons, hydrogen sulphide, and carbon dioxide, which will overcome several of the disadvantages of known methods.